Grouping time

Hello.
Suppose you have a table in an Oracle 10g database with just 2 columns: TODAY_D(date datatype) and FLAG(char(1) for Y or N).
There are a few dozens of records.
Then someone asks you to report quantity of rows (not Yes or Nos, but rows) for each 15 minutes.
Say,
0:00 - 0:15 => 5 records
0:15 - 0:30 => 3 records
..
23:45 - 24:00 => 2 records
How to solve this ?
Tks.